A conventional variable frequency medium voltage drive typically is employed to drive a medium voltage motor for the control of 5 MW to 75 MW loads or processes. The conventional medium voltage drive usually operates at medium voltages of 4.16 kV to 13.2 kV as operation at voltages substantially lower than 4.16 kV leads to excessive currents and power losses. While a conventional medium voltage drive can be manufactured using a standard high voltage power semiconductor, the selection and availability of standard high voltage power semiconductors are limited. In addition, conventional high voltage power semiconductors typically are severely lacking in switching speed, thus limiting the switching frequency. In addition, conventional medium voltage drives typically require specialized multi-level circuitry that tends to increase the dollar cost per kva of the drive.
Conventional low voltage drives of 400V-690V typically cost significantly less per kva than medium voltage drives due to higher production volume, technology maturity and market pressures. However, these conventional low voltage drives are not by themselves suited for controlling 5 MW to 75 MW loads or processes.
One conventional medium voltage inverter, described in E. Cengelci et al., A New Medium-Voltage PWM Inverter Topology for Adjustable-Speed Drives, Vol. 35 No. 3 IEEE Transactions On Industry Applications 628-637 (1999), uses three standard three-phase rectifier/inverter modules coupled between a three-phase input transformer and a three-phase output transformer to generate three-phase medium voltage outputs. However, the connections between the rectifier/inverter modules and the output transformer utilize H-bridge converters and single phase loading that require high voltage isolation and appropriate cooling methods.
Therefore, there is a need for a medium voltage drive or power converter that overcomes the problems noted above and others previously experienced for synthesizing medium voltage variable frequency drive outputs.